Multimedia speakers have been widely used in PCs, home entertainment, and business offices or the like, and are used together with soundcards, personal stereos, CD players, DVD players, laptop computers, MP3 players and mobile telephones, etc. Thus, it is necessary to consider the problem of matching between multimedia speakers and different kinds of audio source devices. Common multimedia speakers employ analog line input ports (LINE IN), which directly matches analog signal line output (LINE OUT) of any of the above audio source devices, only if the line input impedance of the multimedia speaker is greater than or equal to the line output impedance of the audio source.
However, since application situations, power sources, output stage or the like of various audio sources are different from each other in design, thus their line output levels varying, peak-to-peak value Vpp of an output voltage of an audio source may be several times that of another. Even for the same one audio source, e.g., a DVD player, the output level in CD playback mode is different from that in DVD playback mode.
In addition, since power P is equal to U2/R, if a maximum gain of a power amplifier and a speaker impedance are constant, and a maximum output voltage is proportional to an input signal voltage, the power output of the amplifier is proportional to the square of the input signal voltage. That is, if the difference of output levels between two audio sources is 5 times, the difference of power output of the amplifer will be 25 times.
Furthermore, since a power amplifier of a audio system has its rated power output, the Total Harmonic Distortion (THD) will rise steeply when the output level exceeds the rated power, resulting in severe impact on sound effect and even chip breakdown due to over-current in the power amplifier as well as permanent damage to voice coil of a driver units due to overheat.
Therefore, in designing of the power amplifier gain of multimedia speakers, it is necessary to make a trade-off between various audio sources that have high or low output levels. Typically, power amplifier circuits with a variable gain are used to implement compatibility of multimedia speakers with high or low levels input from various audio sources, thus preventing severe distortion of and providing protection for the speakers.
The existing negative-voltage feedback circuits and negative-current feedback circuits are directed to use output voltage or current values to avoid severe distortion. However, since a speaker is a inductive load with complicated frequency variation of instantaneous music signals, it is difficult for these feedback circuits to reflect actual power output, distortion and temperature characteristic of the amplifier. Furthermore, due to the expensive and complicated hardware circuits, debugging workload thereof is heavy. In addition, common Automatic Gain Control (AGC) circuits use voltage output as feedback to control amplifier gain. However, they operate in continuous and simplex regulation mode, and therefore severe fluctuation of the sound level may occur.